Converting to an Asymetric Spinnaker

I have put this information together based on information obtained from a number of sources during the last few months as some of the Auckland fleet have experimented with Asymmetric Spinnakers. I hope it comes in useful, some of it has been learnt by mistake with the odd breakage or gear.

If you have any questions, or want assistance in obtaining materials please let me know.

Rob Fordyce (see the Contacts section)

Choosing a Prod Tube

Materials
Boats using Asymmetrics in Auckland have used prods from a number of sources, made out of a variety of materials.

Material	Cost	Weight	Who has them
Carbon	about $30 $100 + GST about $90 + GST (for 10+)	1kg	Ken Fyfe OR North Sports C-Tech (Alex Vallings)
Alloy	$50 + GST	1.15kg	North Sports
Glass	$40 + GST	1.3kg	North Sports

North Sports are a retail shop which supply windsurfer gear in Auckland, the same sort of sections should be able to be brought from your local suppliers. Ken Fyfe obtains windsurf 2nds from Kilwell, these come in a variety of sizes, materials and layups. Because they are 2nds, supply is sporadic, with each one being different.

Alex Vallings (C-Tech) makes custom built prods for the 12ft skiffs. They would be better engineered than a windsurfer tip (i.e. designed not to bend under high compression) and made in full carbon. The price above would be for a bulk order of 10 or more.

Another option we have thought of is to place an advertisment in your local paper, or contact local windsurfing clubs and get some broken windsurfer masts. If there is enough interest in any of these options we will try and arrange something in Auckland, however, with the wind in Wellington there is probably a better chance of getting broken masts from them.

The weights given above are approximates for prods with end fittings made and wire attached ready to fit onto your boat.

Compression and Loading

The main load on the prod is compression, so if you are wanting the prod to stay in one piece bear this in mind. Once fitted to the boat the boat should be able to be picked up and bounced by holding the prod tip and the transom.

For exotic poles the most important strength factor is the carbon/glass lay-up and tube diameter. My initial prod on Night Nurse (game fishing boat outrigger) was more glass than carbon, when it broke we found that it did not have enough longitudinal fibres for stiffness. Windsurfing top-mast’s appear to have a higher longitudinal fibre content and should prove strong enough.

Advice from Alex Vallings is that the layup should include 45degree fibres as this helps to reduce bend due to compression. This is generally not the way a windsurf tip is layed-up because they are designed to bend. Also his advice is that an untapered section should bend less. On the 12’s they are using a 41.5mm inside diameter un-tapered carbon tube, 1.6mm wall thickness about 1.5m long for their number 3 rigs.

When choosing a Carbon Prod, you should be able to see the Carbon/Glass mixture by looking at the colour change on the ends. My latest prod is a Kilwell windsurf tip, about 80% carbon, 20% glass. Its tip diameter is 30mm, root diameter 43mm with a wall thickness of 2.5mm.

Building the prod and fittings

Attaching Stays - Method 1:

The easiest way we have found to attach wire to the prod is to wrap them on with Kevlar and Carbon. This has advantages in that there are no sharp fittings on the outer end of the prod to snag your kite, and it is light and strong. On my latest prod I started by wrapping about 3 layers of kevlar unis (better fibre strength), and finished off with a layer of Carbon so it would look good.

The side stay wires should be attached to the top of the prod to get a better stay geometry, with the bottom stay obviously attached on the bottom. I used some old 3mm (ex side-stay) wire for my prod stays. The prod side-stay wires are crimped together (using Clamp Products from Don), and are tied to the top of the pole with cotton. A crimp is put on the bottom stay and this is also tied to the pole. I slid the saddle (from a stand-up block used for the tack line) under the side stay wires in the hope that it would help hold the saddle on. A thick epoxy filler mix is applied over the wires to reduce sharp angles, holes and to hold things together. Strips of Kevlar Fibres (about 20mm x 500mm at a time) are wrapped tightly around the saddle, pole and wires to hold everything down. This was finished off with some carbon (looks only) and wrapped with masking tape while it was setting. To stop the prod splitting on the boat stem end the tube was wrapped with an additional layer of carbon.

Attaching Stays - Method 2:
When Ken Fyfe makes Cherub prods he threads the wires through small holes in the prod tip, then puts the thimbles on, and attaches them inside the pole to a stainless through bolt. This is quicker than method one, we haven’t tried it but it works for the Cherubs.

Prod Stem-End Plug
On the hull end of the prod we turn down one end of a nylon rod to the same outside diameter as the prod. Then step down the other end so it slide’s snugly inside the prod to a distance of about 25mm, we have not gluded ours to make de-rigging easier. A 2mm slot is cut into the nylon rod to take a 2mm x 20mm stainless tang which gets attached to the stem of the boat. A hole is drilled through the nylon and stainless tang and a ¼" cap-screw (bolt) is inserted to hold it all together. We have countersunk the nylon for the cap-screw head, and melted a stainless nut into the nylon for the bolt to tighten against to reduce snags. The nylon block is shaped slightly on the base so that the pole can be folded down when rigging the boat

Stem Fitting

The stem fitting is constructed from 2mm stainless plate (35mm x 120 mm), and welded together as a T. This T is later converted into a Y when the fitting is bent around the stem head. NZ Cherubs just attach a gooseneck fitting to the stem head, we didn’t like the extra weight of the gooseneck, and the potential for lines to get caught in the gaps.

Tack Block
I initially used a Ronstan Micro-Block on a spring for the tack block, however found that the rope could fall off the block and result in the plastic being cut out by the tack line. I am now using a stainless block on a spring and find that now the block does not wear out but the rope does.

Fitting prod to boat

Stem Head
The stem head where the stainless fitting sits may need to be reinforced to take the compression from the pole. I put a few layers of glass on my stem head because my stem is foam/glass/bog and not very strong in this point. The glass widens the stem and means the stainless plate does not need to be bent so sharply. I also needed to cut some foam out on the stem head, and glass in some ply so that I would have something to screw into when attaching the stainless plate. Bending the stainless plate will involve a good vice and big hammer to get a tight fit. The tighter the better as you want to spread the compression load out a bit. After we had screwed the plate on resin was squirted into the gaps to achieve a really tight fit.

Bottom Stay
At the bottom of the stem (at least 50mm up to meet rule 11.5.2) a small shackle is attached to take the bottom prod supporting wire. I cut a small notch in the stem to counter sink the shackle into, and ground back an 80mm x 40mm patch of the outer hull glass back to foam. With the shackle in place Kevlar unidirectional fibres should be wrapped through the shackle and onto the foam. It is better to use Kevlar than carbon because Kevlar fibres are not brittle and will not snap as easily. About 4-6 layers of Kevlar should be enough. A layer of glass to finish off will make sanding back and fairing easier (Kevlar will not sand!).

Side Stays
The prod side-stay attachment points should be at a point which gives the maximum angle (within reason) from the boat to the prod tip. This point should be near where the existing spinnaker pole barber-hauler lines are located. The attachment should be very strong, we have used stainless straps through bolted on the gunwale.

Angles

An important part of fitting the prod is getting the geometry/angles correct. There needs to be a vertical difference of about 50-60mm between the prod side-stays and top of the prod at the stem. This ensures that the prod does not accidentally invert and is held upwards when there is decent amount of pre-tension in the side stays. If you raise your prod tip abnormally high you risk having luff length problems with your spinnaker, so try and keep within the ballpark.

Rigging
We rigged our prods so the side-stays shackled directly to the chain-plates with no adjustment. The 12ft skiffs tend to leave their stays a few inches short and use thin spectra braid to bring up the rig tension.

If you want to shackle directly to the chain-plate we found it useful to thread the talurits and thimbles up and use vice-grips on each side to hold the tension while the pole was straightened (eyeball through transom) and checked for upwards tension by pushing down at the tip. We hung a half bucket of water on the tip (5kg) which put the pole on the point of inverting while balancing rig tension against tip height. If you have the pole slightly high (but central) you can take up any stretch later by increasing tension in the bottom stay.

Stay Wire
We have used 3mm wire for our stays which is certainly strong enough, however it may be on the heavy side.

Spreaders
At the moment we have not found a compelling reason to add spreaders, obviously this will depend on the prod section you choose. Let me know what you find out.

Spinnaker Bags vs Chute
There is a lot of debate on this point, I think it comes down to personal preference. I am currently going for a chute, Eddie is going for bags.

We were having a lot of trouble trying to get a bi-fold asymmetric down Night Nurses chute. The tube diameter of this chute is 150mm. We have tried putting a small knot in the halyard between the retrieving patches to stop the chute folding in one big mass to make it easier, but the jury is still out on that one.

For a chute your halyard will probably need to be between 21 and 22m long.

Running Rigging

The Tack Line
I attached a Ronstan C-Cleat to my mast gate, and threaded the tack line through it. It seems to work well and means the crew pulls the tack out while the skipper hoists the kite. You need about 7.5m of line if you bring the spinnaker back into cockpit bags, less than this if you retrieve into a chute.

Sheeting
With the Fyfe kites your existing kite turning blocks should be in about the right position (possibly a little aft). Mine are currently 600mm from the transom. There is no real need for Spectra kite sheets, because they are now only sheets and not also a brace. So you can go for something nice for your hands, with a step up in rope size probably also a good idea for the bigger sail. You will need to get bigger sheets, around about 16m should be enough if you sheet 600 from the transom and lead your sheets to turning blocks about 1m further forward in the cockpit. My sheets ended up needing to be about 1.5m longer than for my symetrical kite.

You will also need Ratchet blocks, and they must be automatic. If they are not automatic (i.e. load sensitive, so they switch off when not loaded up) you will be unable to jibe quickly because your sheets will not run freely.